The manufacture of masked polyisocyanates is known and is described in Houben-Weyl, "Methoden der organischen Chemie, " volume XIV/2, pages 61-70. Tertiary alcohols, phenols, acetoacetic ester, malonic acid ester, acetylacetone, phthalimide, imidazol, hydrogen chloride, cyanogen water and .epsilon.-caprolactam are known as blocking-, coating- or masking agents. These masked isocyanates have the unique ability to react like isocyanate at elevated temperatures.
The more acid the hydrogen atoms of the masking group is, the easier the splitting off. Contrary to the free polyisocyanates, with masked isocyanates there can be made mixtures with hydroxyl-group-containing substances and solvents. without a reaction taking place during mixing or dissolving at relative low temperature, i.e., below the splitting (i.e. unblocking) temperature.
Thus, one is in a position with masked polyisocyanates to produce storable mixtures with hydroxyl-group-containing products, such as higher molecular polyesters or polyethers, which only at higher temperature, i.e, at or above the splitting temperature of the adducts, result in a desired isocyanate reaction. They are of considerable economical important for the production of elastomeric products via storable intermediate as well as for the production of coatings, particularly powder coatings, for metal coatings, etc.
However, many of these masked polyisocyanates, when used as cross-linking agents for terminal hydroxyl-group-containing polymers in a specific field, show an unsatisfactory behavior in actual use or they are excluded from this field of application because of the toxicity of the splitting products.